1. Field of the Invention
The present invention relates generally to processing of greensheets used to make ceramic substrates for microelectronic components.
2. Description of Related Art
In the ceramic electronics industry, multilayer ceramic (MLC) technology is typically used to create three-dimensional circuitry in ceramic substrates for microelectronic devices such as integrated circuits and ceramic capacitors. The three-dimensional circuitry in the ceramic substrate is made by applying a conductive material in a circuit pattern on a ceramic/polymer composite sheet. The ceramic/polymer composite sheet is known as a xe2x80x9cgreensheetxe2x80x9d and may have a number of via holes punched in it to allow vertical connection between the conductive material on adjacent sheets. After the vias are punched, the greensheets are screened and patterned by applying a conductive paste into the via holes and along the surface of the greensheet. The screening is typically done utilizing a mask applied over the individual greensheet. The greensheets are then generally stacked in a designated order and laminated together under appropriate heat and pressure to form a laminate which can be handled as a unified structure. To produce the final ceramic material, the laminated ceramic/polymer composite is fired, i.e. heated, to remove the polymer, followed by heating to higher temperature to sinter and densify the ceramic.
MLC technology has developed to incorporate advanced technologies and groundrules,, i.e., triple dense conductors, thin green sheets and large area greensheets. However, in the course of adopting such new technologies, greensheet stability has become a concern. In particular, radial error of the through holes can and does contribute to misalignment of the conductive vias and other features. The use of one or more of these advanced technologies in a multilayer package, where the individually punched and screened sheets are stacked adjacent to each other to form the laminate, is possible only if the via holes can be punched and screened with a small radial error. It would be especially desirable if this radial error were less than 30 microns.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved via-punched and screened greensheet for use with advanced groundrules, thin green sheets and large area greensheets.
It is another object of the present invention to provide an improved greensheet and processing method which reduces radial error of the screened greensheet.
A further object of the invention is to provide a process and system to make greensheet via alignment more precise in stacked greensheet laminates.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of processing greensheets for use as microelectronic substrates comprising providing a greensheet having a width, a length and a thickness, bonding to the greensheet, within the greensheet width and length, a frame adapted to constrain movement of the greensheet within the frame, processing the greensheet and bonded frame, and removing the frame from the processed greensheet. The processing of the greensheet and bonded frame may include punching vias in the greensheet, filling the vias in the greensheet with conductive material, patterning the greensheet by applying conductive paste to the vias and greensheet surface, stacking the patterned greensheet and bonded frame with at least one other patterned greensheet and bonded frame, and laminating the greensheets. The frame is preferably removed from the processed greensheet after laminating the greensheets, and before the laminated greensheets are subsequently sintered.
In another aspect, the present invention provides a method of processing greensheets for use as microelectronic substrates comprising providing a greensheet having at least one active area, bonding to the greensheet a frame that completely surrounds the active area, the frame adapted to constrain movement of the at least one active area within the frame, processing the greensheet and bonded frame, and removing the frame from the processed greensheet. The greensheet may have a plurality of active areas and the frame may comprise a plurality of subframes such that each of the subframes surrounds one of the plurality of active areas.
In a further aspect, the present invention provides a greensheet having a width, a length and a thickness and a frame bonded to the greensheet within the greensheet width and length. The frame is adapted to constrain movement of the greensheet within the frame.
The bonding of the frame to the greensheet may be by lamination or by an adhesive, or by other means. Preferably, the frame has a thickness less than the greensheet thickness.
The frame preferably has a plurality of members subdividing the greensheet into a plurality of areas, with each area being completely surrounded by frame members. The frame may be applied to one side of the greensheet, and pressed into the greensheet side such that that the frame and greensheet side are substantially coplanar. The entire frame may be bonded within the width and length of the greensheet.
The frame may comprise a material selected from the group consisting essentially of metals, polymers, ceramics, cellulosics, and composites of the above. The metal may be molybdenum, stainless steel, nickel, titanium, tungsten and/or aluminum. The frame is preferably made of a material substantially resistant to movement during the processing of the greensheet and bonded frame. The frame may additionally contain an opening to constrain a via in the greensheet.
The present invention also provides a greensheet laminate stack comprising a plurality of the previously described frame and greensheet combinations, with the plurality the greensheets being stacked and laminated.